fix RLIMIT_ constants for mips

The mips arch is special in that it uses different RLIMIT_
numbers than other archs, so allow bits/resource.h to override
the default RLIMIT_ numbers (empty on all archs except mips).
Reported by orc.

arch/mips/bits/resource.h

@@ -0,0 +1,5 @@
+#define RLIMIT_NOFILE  5
+#define RLIMIT_AS      6
+#define RLIMIT_RSS     7
+#define RLIMIT_NPROC   8
+#define RLIMIT_MEMLOCK 9